An AC signal, one whose amplitude cyclically varies with time, requires a finite time to propagate through various transmission media, such as a capacitance, an inductance or an RF transmission line. After propagating through such media, the phase of the AC signal that exits differs from the phase of the same signal as applied to such media, unless the length of the propagation path through the media is exactly one wavelength in length or an exact multiple thereof. When the length is an exact multiple of the wavelength, the signal's phase undergoes a shift of exactly three hundred and sixty degrees or one cycle. The effect is inherent. In circuit applications in which the phase of one ac signal is critical relative to another ac signal, it often becomes necessary to adjust the amount of phase shift that occurs. That is accomplished by placing an inductor and/or capacitance, referred to as phase shifters, in the signal path of one of the ac signals to account for the greater travel time in one portion of the circuit so that the desired phase relationship between the two differently routed signals is restored. Such phase shifters and their function are well understood.
One common phase shifter application is in phasing of transmitting antennas. In a commercial broadcasting system an RF transmitter delivers RF, modulated with voice, data and/or video information, to an antenna. The antenna radiates that RF into the atmosphere and space, through which the RF propagates over great distances to various receiving stations, such as one's home radio and/or television receiver. There the RF is demodulated and the intelligible information is audibly and/or visibly displayed.
Often the intended receiving stations are not symmetrically displaced from the transmitting antenna. That is particularly true for commercial broadcasting systems, where the broadcast licenses granted by the government, divide geographic regions into separate receiving territories serviced by different broadcast stations. It is therefore desired to broadcast the RF only in a particular direction and over a particular territory so as to avoid interference with another broadcast station operating at or near the same broadcast frequency covering an adjacent territory, ensuring that one station's broadcast does not spill over into the territory of the other. To avoid spillover in those situations and for other reasons, the broadcast stations antennas are designed to be directional.
To achieve directionality, an array of antennas is often used, consisting typically of two or four spaced antennas, that are fed by a single transmitter. The broadcast transmitter's output is coupled to each antenna in the array by a transmission line, suitably a co-axial transmission line, and the antennas are thereby separately fed with RF from the broadcast transmitter. The phase of the RF at each antenna is adjusted in accordance with known phasing principles to differ in such a way as allows the antenna array to produce a well defined directional characteristic or territorial "footprint".
Phase shifters for that application must be capable of handling high power levels of RF. They also must permit some adjustment, since the antenna array when first installed will rarely require the exact amount of phase shift that an "off the shelf" phase shifter provides. Further, the phase shifter should be capable of adjustment, even while under RF load, during the course of broadcasting, so as not to disrupt programming as well as to permit more accurate field monitoring and measurement of radiation levels received at various distances from the transmitting antennas.
At present high power phase shifters that accomplish that function incorporate capacitors and variable inductors. The phase is adjusted by moving the tap on a large inductor, and this is normally done with the transmitter off. Until the present the use of a variable length transmission line to change the phase is not known.
An object of the present invention is to provide a high power adjustable phase shifter in which the propagation time through the phase shifter is adjusted while power is flowing through the line.
A further object of the present invention is to provide a new and easily manufactured high power RF phase shifter that permits easy adjustment of the phase shift characteristic, without the necessity of employing adjustable inductors.
And an ancillary object of the invention is to provide new compositions of dielectric material particularly useful in achieving requisite phase velocity levels in high power RF phase shifters.